The ability of the fuel element to operate at a high power density is dependent to a large extent upon the nature of the bond existing between the fuel matrix and coolant tubes extending there-through, particularly where the cladding is a cooling surface. Large temperature drops with resultant losses in efficiency and possible physical damage to the matrix and coolant tubes occur where no metallurgical bond exists between the fuel matrix and coolant tubes.
The complete fuel-matrix enclosure, which comprises the coolant tubes, header plates, and cladding must be of the highest integrity so as to avoid losses of fission products at the extreme operating conditions experienced by the fuel element.
One method for metallurgically bonding a complete leak-tight metal enclosure to a matrix-type fuel element penetrated longitudinally by a multiplicity of coolant channels is described in copending application of common assignee Ser. No. 472,759. In that application, the metallurgical bond is effected by providing a helium leak-tight enclosure of coolant tubes, header plates and other cladding to the fuel matrix, and exposing the entire assembly to a high temperature and pressure gas environment to effect a diffusion bond between the matrix and enclosure. It has been found that defects often develop in the coolant tubes during the bonding operation, however, due to the tubing expanding into voids existing in the wall of the coolant channels or through the aggravation of already existing cracks inside the tubing which tend to enlarge as the tubing is expanded within the coolant channels.
Exact hydraulic diameters cannot be readily achieved in the coolant tubes when they are expanded within the coolant channels against the fuel matrix. The resulting variations in hydraulic diameters result in coolant flow and temperature variations in an operating fuel element.
The large number of individual leak-tight welds, which are necessary according to the above described method to provide a leak-tight enclosure which permits high pressure gas within the coolant tubes during a bonding operation, increases the cost of fabrication and the probability of failure of the bonding operation due to leaks in one or more of the welds.
It is, accordingly, a general object of the invention to provide a method for metallurgically bonding a complete leak-tight metal enclosure to a matrix-type fuel element which is penetrated longitudinally by a multiplicity of coolant channels.
Another object of the invention is to provide a method for metallurgically bonding a complete metal enclosure to a matrix-type fuel element which is penetrated longitudinally by a multiplicity of coolant channels wherein the deleterious effect of voids in the coolant channel walls on the coolant tubes disposed within the channels is largely obviated.
Still another object of the invention is to provide a method for metallurgically bonding a complete leak-tight metal enclosure to a matrix-type fuel element which is penetrated longitudinally by a multiplicity of coolant channels wherein a minimum number of welds are required.